1. Field of the Invention
The invention relates to a thermometer which includes a 3-terminal high impedance temperature sensitive element. More specifically, the invention relates to such a thermometer which also includes an operational amplifier ratio bridge analogous to a transformer bridge or a hybrid transformer-operational amplifier ratio bridge. In the inventive thermometer, the direct impedance of the 3-terminal high impedance sensor is measured with the operational amplifier ratio bridge or the hybrid transformer-operational amplifier ratio bridge in such a manner that the resistance of the leads and the stray impedances associated with the sensor and its leads do not significantly influence the repeatability and accuracy of the measurement.
2. Description of Prior Art
The use of 3-terminal capacitors as temperature sensitive elements in thermometers has been taught in U.S. Pat. No. 3,759,104, Sep. 18, 1973, Canadian Patent 914,449, Nov. 14, 1972 and British Patent 1,260,730, May 27, 1972 all of which were issued to the inventor herein, and all of which are corresponding patents.
Also known in the art are U.S. Pat. Nos. 3,754,442, Arnett, Aug. 28, 1973 and 4,095,469, Yamada et al, Jun. 20, 1978.
The '469 patent discloses such a thermometer for a temperature measuring apparatus which uses a detecting coil which is placed in a metallic body in which the temperature is to be measured in order to provide an impedance change to measure the temperature of the metallic body. An oscillator is also used in the circuitry of the '469 patent.
The '442 patent describes a temperature measuring and indicating system which provides an output signal which is a linear function of the temperature sensed by a resistive sensing element which is excited by operational amplifiers. A current generator induces a signal in the resistance sensor element which is proportional to the difference between a given reference temperature and the temperature sensed.
It should be noted that the third terminal of a 3-terminal capacitor refers to the electric shield which surrounds the other two electrodes thereby fixing the direct impedance between these first two electrodes. The term, 3-terminal capacitor, also implies that the leads to the first two electrodes are shielded from each other which is normally achieved by extending the shield to surround at least one, but usually, both of their leads. Moreover, from an experimental point of view, a shielded capacitor can be considered to be a 3-terminal capacitor only when connected in special circuits such as transformer bridges in such a manner that the stray capacitances of the first two electrodes and their leads do not affect to any significant extent the measurement of the direct capacitance. For example, a transformer bridge can be used to measure the direct capacitance of, say, 10 pF of a 3-terminal capacitor with a precision of one part per million, or better, in the presence of stray capacitances greater than 100 pF. On the other hand, such a capacitor becomes, in effect, a two terminal component if placed in a typical measuring circuit such as a Wheatstone bridge or when combined with an inductor to produce a resonance circuit. Under such conditions, the direct and stray capacitances appear in parallel so that the direct capacitance, if small, is swamped by the usually much larger stray capacitances.
The concepts of 3-terminal capacitors and transformer bridges are explained in considerable detail in the book authored by B. Hauge and T.R. Foord entitled "Alternating Current Bridge Methods" published by Pitman Publishing Corporation (N.Y.) in 1971. In the following discussion, the term, transformer bridge, will be used to include those bridges often referred to as inductive bridges, transformer ratio bridges, and current bridges, and will also include any other similar bridge based on the properties of inductively coupled ratio arms.
Some of the advantages of 3-terminal capacitance (3TC) thermometers are:
i. The physical size of the sensor can be made very small PA1 ii. By a proper choice of the capacitor dielectric and the insulators and conductors for the leads, shielding, and electrodes, sensors can be constructed to cover the range from less than 0.1 K to approximately 2000.degree. C. even in hostile environments. PA1 iii. The internal dissipation of the sensor can be rendered negligible. PA1 iv. A precision (repeatability) of better than 0.01.degree. C. can be achieved over a wide temperature range. If desired, this precision can be increased to 10.sup.-6 .degree. C. over a temperature range of a few degrees. PA1 v. The sensor is not normally affected to any appreciable extent by direct electric and magnetic fields nor by alternating electromagnetic fields. PA1 vi. Within wide limits, the accuracy of the thermometer is not appreciably affected by the electric resistance of the leads connecting the sensor to the transformer bridge and detector, nor by the capacitive reactance and the conductance between the leads and their shield. PA1 vii. The thermometer is unaffected by thermal electromotive forces in the leads, transformer bridge, and detector. PA1 i. The transformer bridge normally used to measure the capacitance and dissipation of the sensor is bulky, heavy and expensive. PA1 ii. In the case of remote sensing, where the shielded leads connecting the sensor to the bridge and detector are tens and hundreds of meters in length, the small reactance of the stray capacitance between the central conductor and its shield can exceed the limits mentioned in point vi. above and can overload the transformer and short circuit the detector input thereby reducing both the accuracy and sensitivity of the device. Additional error can be introduced by the phase shift and attenuation caused by the capacitive reactance of the shielded leads shunting the resistance of these same leads. Although the reactance of the stray capacitance of the shielded leads can be increased by reducing the frequency of the oscillator activating the transformer bridge, other experimental problems arise when the frequency is reduced below a certain limit, say, at 10 Hz. PA1 iii. Even with the most refractory dielectrics, the dissipation of a 3-terminal capacitive sensor increases at sufficiently high temperatures to such an extent that the component can be considered to be a 3-terminal resistor rather than a 3-terminal capacitor. PA1 1. A 3THI thermometer, based on the accurate measurement by means of an OAR bridge of the temperature sensitive impedance of a high impedance sensor, can be significantly more portable and economic than a 3TC thermometer, which uses a transformer bridge to measure the direct capacitance of the sensor. Using commercially available components, the measuring apparatus can be reduced in size and weight to about those of hand held (pocket) calculators. Using well known microelectronic techniques, the size and weight can be reduced still further by combining the discrete amplifiers and resistors. PA1 2. The shielded leads of the 3THI sensor can be hundreds of meters in length without significantly reducing the precision and accuracy of the thermometer inasmuch as an OAR bridge can, in principle, be operated at arbitrarily low frequencies. PA1 3 The use, in addition to 3-terminal capacitors, of 3-terminal high impedance resistor-capacitors and resistors as sensors permits a greater flexibility in design and a greater choice of materials in the construction of the sensors. In addition to insulators, the suitable sensing materials include ionic conductors, semiconductors, and, at high temperatures, refractory metals such as tungsten, molybdenum, niobium, tantalum their alloys. PA1 an operational amplifier ratio bridge including; PA1 at least one arm, said arm including; PA1 a high impedance 3-terminal impedor being either a 3-terminal capacitor or a 3-terminal resistor or a 3-terminal resistor-capacitor including a first and second electrode each electrode having a lead, said leads being shielded from each other, a temperature sensitive material interposed between said first and second electrodes, a third shielding electrode having a lead, said third electrode being insulated from said first and second electrodes and said leads, said third electrode surrounding and shielding said first and second electrodes and the interposed material for fixing the direct impedance between said first and second electrodes so that said direct impedance varies with temperature as a well defined function thereof, the magnitude of said direct impedance being sufficiently great that the resistance of said leads can be considered to be negligibly small, said 3-terminal impedor comprising the sensing element of said thermometer; PA1 a first operational amplifier whose gain has been accurately determined and stabilized by negative feedback and whose output is connected to said lead of said first electrode of said 3-terminal impedor, the output impedance of said first operational amplifier being reduced by said negative feedback so as to be negligibly small compared to the magnitude of said direct impedance of said 3-terminal impedor and compared to the magnitude of the stray impedances of said first electrode and said lead; PA1 a voltage source providing a voltage of known and stable frequency to the input of said first operational amplifier, said frequency being sufficiently low so that the resistances of said leads of said first and second electrodes become negligibly small compared with the magnitude of the stray capacitive reactances of said leads; PA1 a detecting operational amplifier whose gain has been accurately determined and stabilized by negative feedback and whose inverting input is connected to said lead of said second electrode of said 3-terminal impedor, the input impedance of said detecting operational amplifier being reduced by said negative feedback so as to be negligibly small compared to the magnitude of said direct impedance and compared to the magnitude of the stray impedances of said second electrode and of said lead of said second electrode; PA1 a meter for measuring the real part or the imaginary part or the absolute value of the ratio of the amplitude of the output voltage of said detecting operational amplifier to the amplitude of the voltage signal of said voltage source such that moderate changes in the magnitude of said voltage signal do not significantly change the result of said measurement; PA1 wherein, the temperature which determines the direct impedance of said 3-terminal impedor is calculated from the measured values of said parts of said ratio of said amplitudes.
It is believed that no other type of thermometer has all the above advantages.
However the 3TC thermometer suffers from the following disadvantages:
With respect to the use of terms, it should be noted that no substance is a perfect insulator. Above OK, any so called insulator or dielectric conducts to a certain extent, the conductance often increasing as an approximately exponential function of the temperature. If the conductance is primarily electronic in character, then, above a somewhat arbitrarily defined temperature, a dielectric is usually referred to as a semiconductor. For example, pure germanium and silicon are very good insulators at cryogenic temperatures but are considered to be semiconductors at room temperature. On the other hand, if the conductance is primarily ionic in character, a material is normally considered to be an ionic conductor above a vaguely defined temperature which depends on the frequency of the alternating electric current; and a dielectric below this temperature. In this patent, the terms dielectric and insulator are taken to be synonymous.
With respect to the terms, capacitor and resistor, it should be noted that a real, linear, and passive electric component can be represented mathematically as a combination in parallel and/or in series of at least one ideal resistor, one ideal capacitor, and one ideal inductor. This mathematical representation is not unique and one or another may be chosen as a matter of convenience. In this invention, it has been found convenient to represent each passive component as a parallel combination of an ideal resistor and an ideal capacitor whose values vary with temperature and, to a lesser extent, with frequency. At the frequencies at which the measuring apparatus operates, the inductive contribution to the impedance of the sensor and other passive components used in this invention can be ignored without introducing any significant error.
If, in the parallel representation, the magnitude of the capacitive susceptance is much greater than the conductance, the component is referred to as a capacitor; if the conductance is much larger than the magnitude of the susceptance, the component is referred to as a resistor. In this invention, a component whose susceptance and conductance are approximately equal in magnitude is referred to as a resistor-capacitor. In addition, the term impedor will refer to any component that may be represented by an ideal resistor and an ideal capacitor in parallel; that is, an impedor will signify an almost ideal resistor, an almost ideal capacitor, or a resistor-capacitor.
As in the case of a 3-terminal capacitor, the shield of a 3-terminal impedor fixes the direct impedance between the first two electrodes. Furthermore, from an experimental point of view, a shielded impedor can be considered to be a 3-terminal impedor only when connected in special circuits such as transformer bridges in such a manner that the stray impedances of the electrodes and their leads do not affect in any significant manner the measurement of the direct impedance even when the former are very much smaller than the latter. Otherwise, a shielded impedor becomes, in effect, a 2-terminal component.